Coating of copper and silver air bridge structures to improve electromigration resistance and other applications

ABSTRACT

An improved electrical interconnect for an integrated circuit and methods for providing the same are disclosed. The electrical interconnect includes an air bridge extending through a gaseous medium so as to reduce the capacitance of the interconnect. The air bridge is supported at a first and second end such that the air bridge is suspended above the substrate. The air bridge comprises a highly conductive material, such as silver, so as to provide the air bridge with a reduced resistivity. To inhibit gaseous medium from contaminating the air bridge, the air bridge further comprises an adherent coating interposed between the air bridge and the gaseous medium. A method of forming the electrical interconnect is also disclosed, wherein, prior to forming the adherent coating, the conductive material is processed so as to form fewer grain boundaries, which enhances the electrical properties of the air bridge.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to integrated circuits and, in particular,relates to miniaturized electrical interconnects having reducedresistance and capacitance.

2. Description of the Related Art

To provide improved performance, manufacturers of integrated circuitdevices continually strive to increase circuit density. Such devices aretypically formed on a semiconductor substrate, such as a silicon wafer,and comprise a large number of miniaturized circuit elements. Theseelements, which include transistors, diodes, capacitors, and resistors,are usually disposed within or adjacent the substrate and define aplurality of circuit nodes. To combine the circuit elements into auseful electronic circuit, integrated circuit devices typically includea plurality of conducting paths that link the circuit nodes in apreferred manner. Typically, the conducting paths are provided byelectrical interconnects comprising metallic wires including, forexample, wires made of aluminum or aluminum alloy that are embedded inan insulating layer such as a layer of insulating SiO₂.

However, as circuit density is increased, problems associated withconventional electrical interconnects are becoming more apparent. Inparticular, a higher density device having an increased number ofcircuit elements will likely require an even greater increase in thenumber of electrical interconnects. Consequently, the electricalinterconnects will need to have a reduced thickness and adjacentinterconnects will need to be spaced more closely together.

Unfortunately, such dimensional reductions tend to increase theresistance of individual interconnects and increase the capacitancebetween adjacent interconnects, thereby possibly increasing signalpropagation delays and signal cross-talk. In particular, electricallycharged adjacent conductors acts as the plates of the capacitors. As thedistance between adjacent conductors decrease, the resulting capacitanceincreases. This resulting increase in capacitance slows propagation ofsignals as the capacitance must be overcome prior to propagation of thesignal along the conductor. Hence, while it is desirable to increasedevice density on integrated circuits, considerations such as these poseproblems for maintaining or improving circuit performance.

To improve the conductivity of interconnects, it has been suggested thatcopper metallurgy be substituted for the aluminum metallurgy that is nowtypically being used. Advantageously, copper metallurgy interconnectsare viewed as having increased conductivity and thus less resistance.The lower resistance of interconnects of this metallurgy could allow theuse of smaller dimensions of interconnects thereby facilitating theincrease of device density on the integrated circuit. However, severalpotential problems have been encountered in the development of thisproposed metallurgy. One of the main ones being the fast diffusion ofcopper through both silicon and SiO₂. Fast diffusion of copper intosilicon or silicon oxide results in diffusion of the conductiveinterconnect into the surrounding materials which can damage deviceperformance or can result in short circuits occurring between adjacentinterconnects.

To decrease capacitive loading, it has been suggested that theinterconnects could be embedded in a solid insulating medium other thanSiO₂, such as a polymer comprising fluorinated polymide. However, as inthe case of SiO₂, an incompatibility problem with copper metallurgy hasbeen found. In the case of polyimide, and many other polymers, it hasbeen found that the polymer, during the curing, reacts with copperforming a conductive oxide CuO₂, which is dispersed within the polymer.This then raises the effective dielectric constant of the polymer and inmany cases increases its conductivity. Hence, there have been numeroussuggested approaches towards solving the problems of capacitive couplingand increased resistance occurring as a result of a need to formulatesmaller dimensioned interconnects that are spaced closer together. Aprimary difficulty that results is the relative incompatibility of lowerresistance materials with the surrounding insulating material.

Silver is one of the best conductors, in that it has the lowest specificresistivity of any metal or alloy. Furthermore, a vacuum is the ultimatedielectric, with air being nearly as good. However, the use of a vacuumintroduces additional problems or complexities to the device. The firstbeing the low heat conductivity of the vacuum and the second being thecost of the package required to maintain the vacuum. Air, which hassomewhat better thermal conductivity, has its own problems in that bothcopper and silver react with air to form oxides or other compounds.Alternatively, gold is known to be quite environmentally stable. Howeverit's specific resistivity is higher than that of copper and silver.

To address the problem of increased capacitance, interconnectscomprising an air bridge have been developed as described in U.S. Pat.No. 5,891,797. The air bridge is a length of conducting material thatextends from a first supported end to a second supported end through anair space such that the bridge is substantially surrounded by air.Consequently, because air has a dielectric constant that issubstantially less than that of SiO₂, the capacitance between adjacentinterconnects is reduced.

However, because the air bridge tends to sag under its own weight, thelength of the air bridge is a possible limiting factor. In particular,because the air bridge is only supported at its first and second ends,gravitational forces acting on the air bridge when the bridge ishorizontally disposed cause the air bridge to sag such that theunsupported middle of the air bridge is deflected downward with respectto the first and second ends. Because the degree of sagging increases asthe length of the bridge is increased, the length of the air bridgecannot exceed that which would cause the air bridge to break or comeinto contact with another conductor of the device.

According to classical mechanics for simple air bridge structures, thecenter of the bridge is deflected downward with respect to the supportedand constrained ends by an amount δ given by${\delta = \frac{\rho\quad L^{4}}{32h^{2}E}},$wherein ρ is the mass per unit volume of the air bridge, L is the lengthof the air bridge, h is the height of the air bridge, and E is themodulus of elasticity of the air bridge. Consequently, aside from thegeometric factors L and h, the deflection δ is proportional to the ratioof (ρ/E). Thus, an air bridge formed of a material having a reducedratio of (ρ/E) will experience less sagging. If the ends of the bridgeare not considered to be constrained then$\delta = {\frac{5\rho\quad L^{4}}{32h^{2}E}.}$This is the worst case assumption.

Resistivity Elastic Modulus Mass Density Material (nΩm) (GPa) (Mg/m3)ρ/E Copper 16.7 128 8.93 0.0698 Silver 14.7 71 10.5 0.148 Gold 23.5 7819.3 0.247 Aluminum 27.5 70 2.7 0.039

The table above illustrates the physical properties of possible airbridge materials. Both copper and silver have resistivities that aresubstantially less than that of aluminum and, thus, would provide airbridges with reduced resistance. Because copper has a ratio of (ρ/E)which is less than that of silver, a low resistance bridge comprisingcopper would experience less sagging and, thus, would be more suitablefor applications that require bridges having extended lengths.Alternatively, because silver has a resistivity less than that ofcopper, a bridge comprising silver would be more suitable forapplications that require reduced resistance. However, as was pointedout previously, both copper and silver are susceptible to environmentaldegradation in an air environment.

Gold also has a resistivity less than that of aluminum. Furthermore,gold is not susceptible to environmental degradation in an airenvironment. However, because the resistivity of gold and the ratio of(ρ/E) of gold are substantially larger those of silver or copper, abridge formed of gold would have a relatively large resistance and wouldexperience a relatively high degree of sagging.

Various processing techniques may also contribute to the effects ofdevice reliability and environmental degradation. For one, annealing isa process involving heating and cooling of a mechanically work-hardenedregion, which is designed to effect the microstructure of crystallinematerials. Annealing typically softens work-hardened microstructures byrelieving residual stress caused by mechanical processes, such aspolishing and/or grinding. Additionally, for sub-micron structures,chemistry is a substantially important variable for establishing highelectrical conductivity in conductive interconnects. The working ofmechanical processes may significantly decrease electrical conductivityand retard grain development.

As is known in the art, abnormal grain development may be associatedwith a duplex grain structure caused by the dissolution of oxides duringa high temperature annealing process. The propensity for graincoarsening and duplex grains may be attributed to excessive solutiontemperatures and oxygen concentrations. Unfortunately, coarse grainsformed during high temperature anneals may remain present after cooling.In addition, the rate of cooling from high temperature anneals may alsodetrimentally influence the mechanical properties of materialscomprising high levels of impurities. Furthermore, rapid cooling mayalso result in substantially high, non-equilibrium levels of impuritiesin solid solution. Alternatively, slow cooling may allow for theinteraction between impurities and oxygen, which may lead to subsequentprecipitation from solid solution. Typical high temperature annealingtechniques may be considered harmful and may detrimentally effectchemical, electrical, and mechanical properties of crystallinematerials, wherein localized inhomogeneities may change with deformationand thermal history, metal purity, and oxygen content.

The reduction in conductor size introduces additional problems as thesurface to volume ratio increases, as it must with the reduced conductorsize, the specific electromigration resistance decreases. This is adirect result of the fact that the surface diffusion rate is higher thanthe grain boundary diffusion rate which is higher than the “bulk” rate.As the relative surface area increases the surface diffusion rate, whichmay be up to two orders of magnitude greater than the bulk rate,.becomesmore and more significant.

From the foregoing, therefore, it will be appreciated that there is aneed for an improved air bridge structure for an integrated circuit thatnot only provides a relatively small resistance but also is extendableover relatively large distances. It should also be appreciated thatthere exists a need to improve processing methods associated with airbridge structures for the purpose of increased reliability.

SUMMARY OF THE INVENTION

The aforementioned needs are satisfied by one aspect of the presentinvention which discloses a method and device for forming an electricalinterconnect comprising an air bridge structure for electricallyconnecting at least two circuit elements in an integrated circuit. Inorder to reduce the electromigration rate of the copper or any othersub-micron conductor it will be necessary to find ways to reduce theeffect of surface diffusion on the electromigration rate. This can beaccomplished by coating the surface of the conductor with a thin highlyadhesive coating, which has a low solubility in the base conductor. Orif the coating has a significant solubility it must have such a lowdiffusion rate into the conductor at the processing and use conditionsthat it does not penetrate the conductor, during the time of service.For a copper conductor, a thin zirconium film can be deposited byselective plating or CVD on to the conductor, after the last hightemperature step. The coating material must form an adherent layer uponthe surface such that diffusion along the boundary between the coatingand the base material is significantly less than the surface diffusionrate of the base material.

The aforementioned needs may be satisfied by a method of forming an airbridge structure between a first and second circuit component on asubstrate. In one embodiment, the method may comprise forming a supportstructure on the substrate, forming vias in support structure above thefirst and second circuit components, and depositing a conductive layerso as to form vertically extending legs in the vias and a laterallyextending member between the upper portions of the vertically extendinglegs in a manner so as to electrically interconnect the first and secondcircuit components, wherein forming the laterally extending memberresults in an increased resistivity through the air bridge structure.The method may further comprise removing the support structure so as tosuspend the laterally extending member above the substrate between thefirst and second circuit components via the vertically extending legs,wherein removing the support structure results in an increasedresistivity through the air bridge structure, and processing the airbridge structure by re-crystallizing the laterally extending member andthe vertically extending legs, which results in a decreased resistivitythrough the air bridge structure.

In one aspect, the depositing the conductive layer may includedepositing a material with a line resistivity of at least less than thatof aluminum. Also, depositing the conductive layer may includedepositing a material with a ratio of mass density over modulus ofelasticity (E/ρ) that is at least greater than gold. In addition,re-crystallizing the laterally extending member may comprises coalescingthe grain boundaries in a manner so as to form fewer grain boundaries.Coalescing the grain boundaries may occur at room temperature.Coalescing the grain boundaries may include performing a heat treatment.Coalescing the grain boundaries may improve the electrical properties ofthe conductive layer. Improving the electrical properties of theconductive layer includes enhancing the electromigration resistance ofthe conductive layer. Improving the electrical properties of theconductive layer includes enhancing the diffusion resistance of theconductive layer.

In another aspect, the method may further comprise annealing the airbridge structure. Also, forming the laterally extending member includesplanarizing the conductive layer using a CMP process. Forming aconductive layer comprises depositing a least one of copper, silver, andgold. In addition, the method further comprises forming an adherentcoating on the air bridge structure. Forming the adherent coating on theair bridge structure includes depositing at least one of titanium,zirconium, and hafinium on the air bridge structure.

The aforementioned needs may be satisfied by a method of forming an airbridge structure between a first and second circuit component on asubstrate. In another embodiment, the method may comprise forming asupport structure on the substrate, forming vias in support structureabove the first and second circuit components and depositing aconductive layer so as to form vertically extending legs in the vias anda laterally extending member between the upper portions of thevertically extending legs in a manner so as to electrically interconnectthe first and second circuit components, wherein forming the laterallyextending member exposes grain boundaries adjacent the surface of thelaterally extending member resulting in an increased resistivity throughthe air bridge structure. The method may further comprise removing thesupport structure so as to suspend the laterally extending member abovethe substrate between the first and second circuit components via thevertically extending legs, wherein removing the support structureresults in forming grain boundaries in the laterally extending memberand the vertically extending legs, which increases resistivity throughthe air bridge structure, and processing the air bridge structure bycoalescing the grain boundaries so as to form fewer grain boundaries,which results in a decreased resistivity through the air bridgestructure.

The aforementioned needs may be satisfied by a method of forming anelectrical interconnect for an integrated circuit having a substratewith at least two semiconductor components. In still another embodiment,the method may comprise forming a bridge structure having a crystallinemicrostructure by laterally extending a first material between the atleast two semiconductor components in a manner so as to suspend thefirst material in a gaseous medium above the substrate of the integratedcircuit, wherein forming the first material produces grain boundaries inthe crystalline microstructure and re-crystallizing the first materialin a manner so as to form fewer grain boundaries in the crystallinemicrostructure, wherein forming fewer grain boundaries improves theelectrical properties of the first material. The method may furthercomprise insulating the first material with a second material so as tosubstantially reduce environmental degradation of the first material andapplying a heat treatment in a manner so as to strengthen the adhesivebond between the first and second material, wherein the heat treatmentfurther improves the electrical properties of the first material.

The aforementioned needs may also be satisfied by an integrated circuitdevice comprising a semiconductor substrate, at least two circuitcomponents formed on the semiconductor substrate and spaced distallyapart, and at least one bridge structure laterally extending between theat least two circuit components in a suspended manner above thesemiconductor substrate so as to electrically interconnect the at leasttwo circuit components, and wherein the at least one bridge structure isdisposed adjacent a gaseous medium so as to reduce the capacitance ofthe at least one bridge structure, and wherein the at least one bridgestructure comprises a reduced grain boundary component that is processedso as to improve the electrical properties of the at least one bridgestructure.

In one aspect, the at least one bridge structure may comprise aconductive material. The conductive material may include copper. Theconductive material may include silver. The conductive material mayinclude gold. The conductive material may comprise a resistivity lessthan that of aluminum. The conductive material may comprise a ratio ofmass density over modulus of elasticity (E/ρ) that is greater than gold.The at least one bridge structure may be coated with an insulatingmaterial so as to improve the environmental degradation resistance ofthe at least one bridge structure. The insulating material may includeat least one of titanium, zirconium, hafnium, chromium, and vanadium.The gaseous medium may comprise air. The gaseous medium may comprise anon-conductive fluid. The nonconductive fluid may include anon-conductive gas. The non-conductive gas may include carbon-dioxide.The gaseous medium comprises an insulating material. The insulatingmaterial may comprise at least one of a polymer, a foamed polymer, apolymide, a foamed polymide, an inorganic material, and a porousinorganic material.

From the foregoing, it should be apparent that the electricalinterconnect of the present invention and methods of providing the sameprovide many advantages over interconnect known in the art. Inparticular, because the bridge section of the interconnect is disposedadjacent an air space instead of a solid insulating material, the bridgemay comprise a reduced capacitance. In addition, because the material ofthe bridge structure is more conductive than that which is used intypical interconnects, the interconnect of the present invention may beformed with an increased length and a reduced cross-sectional area.Moreover, processing the bridge structure so as to coalesce grainboundaries prior to applying the adherent coating may enhance theelectrical properties of the bridge structure such that the bridgestructure comprises a lower resistivity. These and other objects andadvantages of the present invention will become more apparent from thefollowing description taken in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a schematic diagram of an integrated circuit deviceaccording to one aspect of the present invention, the device comprisingat least one electrical interconnect having an air bridge structure;

FIG. 2 illustrates a cross-sectional schematic diagram of the air bridgeof FIG. 1 as seen along a y-axis;

FIG. 3 illustrates a cross-sectional schematic diagram of the air bridgeof FIG. 1 as seen along an x-axis;

FIG. 4 illustrates a flow chart of one embodiment of a method of formingthe electrical interconnect of FIG. 1;

FIG. 5 illustrates a cross-sectional schematic diagram of one embodimentof the electrical interconnect of FIG. 1 in a partially fabricated stateaccording to the method of FIG. 4;

FIG. 6 illustrates a flow chart of one embodiment of a method of formingadjacent electrical interconnects having overlapping air bridgesections;

FIG. 7 illustrates one embodiment of a cross-sectional schematic diagramof the electrical interconnects of FIG. 1 in a partially fabricatedstate according to the method of FIG. 6;

FIG. 8 illustrates one embodiment of an integrated circuit having an airbridge electrical interconnect interposed between two circuit elements;

FIGS. 9A-9H illustrate one embodiment of forming the air bridgeelectrical interconnect of FIG. 8 with enhanced electrical propertiesincluding improved electromigration properties.

FIG. 10 illustrates one embodiment of a process flow for forming the airbridge electrical interconnect in FIGS. 8, 9 with enhanced electricalproperties.

DETAILED DESCRIPTION IF THE PREFERRED EMBODIMENT

The illustrated embodiments of the present invention comprise aminiaturized electrical interconnect having improved operatingcharacteristics and methods for providing the same. The electricalinterconnect includes a bridge section surrounded by air, referred tohereinbelow as an “air bridge”, so as to reduce the capacitance of theinterconnect. Air bridges are also described in U.S. Pat. No. 5,891,797which is incorporated herein by reference in its entirety. As will bedescribed in greater detail below, air bridges formed in accordance withthe various aspects of the present invention are provided with reducedresistance and a reduced tendency to sag, thereby enabling them to havea reduced cross-sectional area and to extend across larger distances.

Improved electrical interconnects formed according to the methods of theillustrated embodiments are particularly useful in the manufacture ofultra-high density integrated circuit devices, such as a Dynamic RandomAccess Memory (DRAM), a microprocessor, or a Digital Signal Processor(DSP). It should be understood, however, that the methods describedhereinbelow could be used in any application or structure in which it isdesirable to include improved miniaturized electrical interconnects.Furthermore, the methods of the present invention are particularlywell-suited for providing improved electrical interconnects on or abovea semiconductor substrate, such as a silicon wafer, or substrateassembly, referred to herein generally as “substrate,” used in formingany of a number of conventional integrated circuits.

It should be understood that the methods of the present invention arenot limited to integrated circuits formed on silicon wafers; rather,other types of wafers (e.g., gallium arsenide, etc.) can be used aswell. Thus, the skilled artisan will find application for the processesand materials discussed below for any of a number of devices requiringimproved electrical interconnects.

Reference will now be made to the drawings wherein like numerals referto like parts throughout. FIG. 1 schematically illustrates an integratedcircuit device 30 according to one aspect of the present invention. Theintegrated circuit 30 comprises a plurality of known circuit components,such as transistors, resistors, capacitors and the like, formed in awell known manner. The circuit components are formed within, on, orabove a substrate 32. In one embodiment, the substrate 32 has a planarshape and is aligned in an x-y plane as shown in FIG. 1. The circuitcomponents define a plurality of circuit nodes which are interconnectedby way of a plurality of improved electrical interconnects as will bedescribed in greater detail below.

As schematically illustrated in FIG. 1, the integrated circuit comprisesa first electrical interconnect 34 extending from a first node 36 to asecond node 38 of the integrated circuit 30. The first interconnect 34has an elongate shape which is shown extending in a linear manner alongthe y-axis. However, it will be appreciated that, in another embodiment,the first interconnect 30 could have a different shape and extend in anon-linear manner along virtually any direction with respect to thesubstrate. In one embodiment, the integrated circuit 30 furthercomprises a substantially similar second electrical interconnect 40extending along the x-axis from a third node 42 to a fourth node 44 suchthat the second interconnect 40 overlaps the first interconnect 34 asshown in FIG. 1. While the nodes 36, 38, 42 and 44 are described in thisembodiment as being positioned within the substrate 32, it will beappreciated by a person of ordinary skill that the nodes can actually beformed in an insulating layer positioned over the substrate 32. Hence,the use of the bridge structures described herein should not be viewedas being limited to use with nodes formed in the substrate as they canbe used between nodes formed in or above the substrate 32.

FIGS. 2 and 3 further illustrate the integrated circuit 30 of FIG. 1,wherein FIG. 2 is a schematic diagram corresponding to a view along thex-axis and FIG. 3 is a schematic diagram corresponding to a view alongthe y-axis. The first interconnect 34 comprises a first end section 46extending from the first node 36, a second end section 48 extending fromthe second node 38, and a bridge section 50 extending between the firstand second end sections 46 and 48. The first end section 46 supports afirst end 47 of the bridge section 50 and the second end section 48supports a second end 49 of the bridge section 50. Consequently,gravitational forces acting on the bridge section 50 cause anunsupported midpoint 51 of the bridge section 50 to sag such that themidpoint 51 is downwardly displaced with respect to the ends 47 and 49.As will be described in greater detail below, in one embodiment, thebridge section 50 comprises a material having a reduced ratio of massdensity over modulus of elasticity (p/E) so as to reduce the degree ofsagging.

The end sections 46 and 48 and the bridge section 50 comprise aconducting 5 material that provides a conducting path extending betweenthe first and second nodes 36 and 38. Furthermore, the bridge section 50is disposed in a plane that is outwardly displaced from the plane of thesubstrate. Moreover, the bridge 50 extends through a space 52 having agaseous medium disposed therein such that the bridge section 50 issubstantially surrounded by the gaseous medium. In the preferredembodiment, the gaseous medium comprises air or any other low dielectricgaseous mixtures. Consequently, because air has a relatively smalldielectric constant, the first interconnect 34 is provided with arelatively small capacitance with respect to nearby conducting elementsof the device.

In one embodiment, the first and second end sections 46 and 48 laterallyextend from the respective first and second nodes 36 and 38 and thebridge section 50 longitudinally extends therebetween. However, a personskilled in the art will realize that the methods described herein couldalso be used to form interconnects having an alternative geometry. Forexample, the end sections could extend from the nodes 36 and 38 withlongitudinal components and the bridge section could extend with alateral component. Furthermore, rather than extending along a planedisposed away the substrate, in another embodiment, the bridge sectioncould extend through a trench formed within the substrate such that thebridge section substantially overlaps the plane of the substrate.Moreover, in yet another embodiment, the electrical interconnect couldconsist solely of the bridge section such that the bridge sectionextends directly from the first node to the second node through thetrench formed in the substrate or in an insulating layer formed on thesubstrate.

In one embodiment, the second interconnect 40 comprises a second airbridge 54 which is substantially similar to the air bridge 50 of thefirst interconnect 34. As shown in FIGS. 2 and 3, the second air bridgeextends between laterally disposed first and second end sections 56 and58 through the air space 52 such that the second bridge 54 is disposedabove the first air bridge 50. Thus, because air separates the first andsecond air bridges 50 and 54, the capacitance between the first andsecond interconnects 34 and 40 is reduced. Consequently, independentsignals propagating along the first and second interconnects 34 and 40are less likely to interfere with each other and the speed ofpropagation of signals will be less effected by capacitance.

As shown in FIGS. 2 and 3, the air bridge 50 comprises a core 60extending along its length that provides the air bridge 50 withdesirable electrical and mechanical properties. In particular, topromote conduction along its length, the core 60 preferably comprises ahighly conductive material. Furthermore, to reduce sagging, the core 60preferably comprises a material having a relatively small ratio of massdensity over modulus of elasticity (ρ/E). As mentioned above in thebackground section, materials having relatively low resistivity andrelatively low ρ/E include copper and silver. In one embodiment, thecore 60 comprises copper. In another embodiment, the core 60 comprisessilver.

As shown in FIGS. 2 and 3, the air bridge 50 further comprises a tightlyadherent coating 62, that is deposited on the core 60 and substantiallysurrounds the core 60, such that the coating 62 is interposed betweenthe core 60 and the air of the air space 52. In one embodiment, thepurpose of the adherent coating 62 is to provide the air bridge 50 withimproved electromigration resistance along with desirable environmentalproperties. In another embodiment, the adherent coating 62 comprises aprotective coating that serves as a protective barrier which preventssurface diffusion as well as inhibiting contaminants, such as oxygen,from reaching the core 60. Furthermore, the coating 62 preferablycomprises a material having a low solubility with respect to the core 60that does not readily diffuse into the core and significantly degradethe conductivity of the core 60. Thus, because the core 60 issubstantially shielded from the air space 52, the core 60 is able toinclude environmentally sensitive materials, such as copper or silver,that provide the bridge 50 with reduced resistance and reduced sagging.

In one embodiment, the coating 62 comprises a conducting material, thatreduces surface diffusion and resists air molecules from diffusingtherethrough and enhances conduction along the bridge 50. For example,the coating can include the reactive elements titanium, zirconium orhafnium. If one of the reactive elements is used, zirconium may bepreferred due to its low solubility in both copper and silver.

In another embodiment, the coating 62 comprises an insulating materialthat inhibits air molecules from reaching the core 60. For example, thecoating 62 could comprise an inorganic material such as Si₃N₄.

Reference will now be made to FIGS. 4-5 which illustrate a method 100 offorming an individual electrical interconnect according to oneembodiment of the present invention. As will be described in greaterdetail below, the method 100 essentially comprises forming the core 60of the interconnect 34 and then disposing the coating 62 on the core 60.

As shown in FIG. 4, in one embodiment, the method 100 comprises, in astate 102, forming a temporarily support structure or mandril. Thepurpose of the mandril is to provide a supporting surface that supportsthe bridge section of the electrical interconnect during formation ofthe bridge section. The mandril can be formed from any of a wide varietyof materials that provide the electrical interconnect with temporarysupport and that can subsequently be removed to expose a lower surfaceof the air bridge section.

For example, as shown in FIG. 5, the temporary support structure maycomprise a layer 104 of photoresist which is deposited over thesubstrate 32 using conventional deposition techniques. The photoresistlayer 104 is deposited with a substantially uniform thickness such thatthe substrate 32 is substantially covered by the layer 104 and the firstand second nodes 36 and 38 are disposed under the layer 104. Thethickness of the layer 104 is selected so as to provide a desiredseparation distance between a lower surface 106 of the bridge 50 and anupper surface 108 of the substrate 32.

As shown in FIG. 4, the method 100 further comprises, in a state 120,modifying the mandril so as to expose the first and second nodes 36 and38. In particular, using conventional etching techniques, first andsecond vias 110 and 112 are formed in the mandril that vertically extendfrom an upper surface 114 of the mandril 104 to the respective first andsecond nodes 36 and 38 of the circuit 30 as shown in FIG. 5.

As shown in FIGS. 4 and 5, the method 100 further comprises, in a state130, depositing a conducting layer 132 over the mandril 104 such thatthe conducting layer 132 horizontally extends across the upper surface114 of the mandril between the vias 110 and 112, so as to subsequentlyform the core 60 of the bridge 50, and vertically extends through thevias 110 and contact the first and second nodes 36 and 38 so as toprovide the end sections 46 and 48 of the interconnect 34. Because theconducting layer 132 will eventually become the core 60 of the firstelectrical interconnect 34, the conducting layer 132 preferablycomprises a highly conductive material having a relatively small ratioof (ρ/E), such as silver or copper, thereby providing the electricalinterconnect 34 with a relatively small resistance and a reducedtendency to sag as will be described in greater detail hereinbelow.

As shown in FIGS. 4 and 5, the method 100 further comprises, in a state140, modifying the conductive layer so as to define the shape of thecore 60 of the electrical interconnect 34. For example, the core 60 canbe shaped by employing conventional pattern and etching processes thatleave behind the first and second end sections 46 and 48. verticallyextending from the respective nodes 36 and 38 and also leave behind thebridge section 50 horizontally extending between the end sections 46 and48 as shown in FIG. 5.

However, it will be appreciated that the core 60 of the interconnect 34could be formed in an alternative manner without departing from thespirit of the present invention. For example, in an alternativeembodiment, the core 60 could be formed by defining a trench in themandril, depositing conductive material in the trench, and removingexcess conductive material using a conventional chemical mechanicalplanarization process.

As shown in FIG. 4, the method 100 further comprises, in a state 150,removing the mandril so as to expose the lower surface 106 of the core.In one embodiment, the photoresist layer 104 is removed by exposing thephotoresist layer 104 to a known etchant that selectively removes thephotoresist 104 layer and does not remove the core 60 of theinterconnect 34.

As shown in FIG. 4, the method 100 further comprises, in a state 154,disposing the adherent coating 62 on the exposed surfaces of the core 60of the electrical interconnect 34. In one embodiment, disposing theadherent coating 62 comprises depositing a layer of conductive materialselected from the group comprising the noble metals gold, platinum,palladium, iridium, and the reactive elements titanium, zirconium andhafnium. Furthermore, the conductive material of the coating 62 can bedeposited using a known electroless plating process, or a known chemicalvapor deposition process (CVD), such as Plasma Enhanced Chemical VaporDeposition (PECVD).

In one embodiment, disposing the coating 62 comprises depositing aninsulating material. For example, the insulating material can comprisean inorganic material, such as Si₃N₄, which can be deposited usingPECVD. If diffusion of the coating 62 into the core 60 is a concern,such diffusion can be reduced by not exposing the bridge 50 to elevatedtemperatures. Preferably, the conductive material is deposited so thatthe material only deposits on the core 60.

In one embodiment, the bridge section 50 of the first interconnect 34has a rectangular cross-sectional shape with a width approximately equalto 0.25 microns and a height approximately equal to 0.25 microns.Consequently, the bridge section 50 comprising the copper core 60 isable to span a distance of 0.25 mm with a worst case sagging deflectionapproximately equal to 0.0065 microns. Furthermore, at this length, thebridge section 50 provides a resistance of only 67 ohms. Alternatively,if the core 60 is formed of silver, the bridge section 50 has aresistance approximately equal to 59 ohms and a sagging deflectionapproximately equal to 0.014 microns. In comparison, a similarly shapedaluminum bridge section would provide a substantially larger resistanceapproximately equal to 110 ohms and experience a sagging deflectionapproximately equal to 0.0035 microns.

As can be seen from the above example the limiting factor for LongAluminum Bridge Structures is the line resistivity not the tendency tosag. If 50 ohms were the limit then Copper and Silver would both bemarginally acceptable at this dimension while Aluminum would beunacceptable.

Preferably, the coating 62 has a thickness that substantially inhibitscontaminants, such as oxygen, residing in the air space 52 from reachingthe core as well as adhering tightly to the core such that surfacediffusion is significantly reduced. For example, in one embodiment, thecoating 62 including one of the conducting materials listed above has athickness approximately between 20 Å and 40 Å. In another embodiment,the coating 62 including one of the insulating materials listed aboveand has a thickness approximately between 10 Å and 100 Å.

Reference will now be made to FIGS. 6-7 which illustrate a method 200 offorming a plurality of adjacent electrical interconnects havingoverlapping air bridge sections in accordance with yet anotherembodiment of the present invention. As shown in FIG. 6, the method 200comprises forming the mandril 104 in a state 202, and forming the core60 of the first interconnect 34 above the mandril in a state 204 in themanner described above in connection with FIG. 4.

As shown in FIGS. 6 and 7, The method further comprises, in a state 206,extending the mandril 104 with a second photoresist layer 208 thatcovers the core 60 of bridge section 50 of the first interconnect. Thepurpose of the second layer 208 is to support and elevate a core 41 ofthe second interconnect 40 above the first core 60. The second layer 208includes an upper surface 210 which is displaced above an upper surface212 of the first core 60. The thickness of the second layer 208 isselected so as to provide a desired distance between the upper surface212 of the first core 60 and the upper surface 210 of the secondphotoresist layer 208.

As shown in FIG. 6, the method further comprises forming the secondinterconnect 40, in a state 220, so that the second interconnect 40extends between the third and fourth circuit nodes 42 and 44, of theintegrated circuit (FIGS. 2 and 3). The second interconnect 40 ispreferably formed using the methods described above in connection withFIG. 4; i.e. forming vias in the mandril layers 104, 208, depositing alayer of conducting material over the mandril, and patterning theconducting material. Furthermore, the overlapping core 41 of the secondinterconnect 40 preferably extends along a direction that isperpendicular to that of the first interconnect 34 and 40 so as toreduce capacitive coupling between the first and second interconnects.

As shown in FIG. 6, the method further comprises, in a state 222,removing the mandril. In one embodiment, removing the mandril comprisesremoving the mandril layers 104 and 208 after completing the cores 60and 41 of the respective electrical interconnects 34 and 40. Inparticular, after forming the first and second cores 60 and 41, thefirst and second photoresist layers 104 and 208 are removed in a singleetching process. However, it will be appreciated that, in anotherembodiment, the first madril layer 104 could be removed subsequent toforming the first core 60 in a first etching process and the secondmadril layer 208 could be removed in a separate second etching processsubsequent to forming the second core 41.

As shown in FIG. 6, the method 200 further comprises depositing theadherent coating 62 in a state 224. In one embodiment, the adherentcoating 62 is simultaneously deposited on the cores 60 and 41 of thefirst and second interconnects 34 and 40 in the manner described abovein connection with FIG. 4. The advantage of simultaneously depositingthe coating on both cores 60 and 41 is that fewer processing steps areneeded. However, it will be appreciated that each of the cores 60 and 41could be coated during separate deposition stages without departing fromthe spirit of the present invention.

It will be appreciated that the electrical interconnect and methods forproviding the same of the present invention provide many advantages. Inparticular, because the interconnect includes the air bridge which issurrounded by air, the interconnect is provided with a reducedcapacitance. Consequently, the interconnect is less susceptible to theproblems of signal delay and signal cross-talk. Furthermore, because thecore of the air bridge is formed of highly conductive material, the airbridge is able to have a reduced resistance, thereby further reducingsignal delays. Moreover, because the core of the air bridge is formed ofa material having a relatively low ratio of ρ/E, the air bridge is lesssusceptible to the problems of sagging. Thus, the air bridge is lesslikely to fracture and/or contact adjacent structures of the integratedcircuit, thereby allowing adjacent interconnects to be spaced moreclosely together and span larger distances. Additionally, because thecore of the air bridge is surrounded by the adherent coating, thesurface diffusion rate of the core material will be substantiallyreduced thus increasing the electromigration resistance of thestructure. The oxygen from the air surrounding the air bridge is alsoinhibited from reacting with the core which would otherwise contaminatethe core which could possibly increase the resistance of the core anddecrease the mechanical strength of the core.

FIG. 8 illustrates one embodiment of an integrated circuit 300. Theintegrated circuit 300 comprises a substrate 302 having an upper surface304, a first circuit element 310 having a first mounting region 312, anda second circuit element 320 having a second mounting region 322. Thecircuit elements 310, 320 may comprise generally known transistors orother types of circuit elements, such as resistors and capacitors,having a plurality of mounting regions without departing from the scopeof the present invention. The first and second mounting regions 312, 322function as electrical contact points for the first and second circuitelements 310, 320, respectively. Additionally, the first and secondmounting regions 312, 322 may be formed in a known manner with aconductive material, such as polysilicon, aluminum, copper, or silverusing known metallization and/or deposition techniques, such as CVD anddamascene processes.

In one aspect, the illustrated substrate 302 may comprise a conventionalsilicon wafer, but may generally encompass structures comprisingsemiconductor material, including, but not limited to, bulksemiconductor materials such as a semiconductor wafer (either alone orin assemblies comprising other materials thereon), and semiconductivematerial layers (either alone or in assemblies comprising othermaterials). In addition, the term “substrate” may also encompass anysupporting structures, including, but not limited to, the semiconductivesubstrates described above. Furthermore, when reference is made tosubstrate within the following description, previous process steps mayhave been utilized to form regions, structures, or junctions in or onits base semiconductor structure or foundation.

FIG. 8 further illustrates one embodiment of the formation of anelectrical interconnect 330. In this particular embodiment, theelectrical interconnect 330 comprises an air bridge structure that isformed to laterally extend between the first and second circuit elements310, 320 in a suspended manner above the substrate 302 so as toelectrically interconnect the at least two circuit elements 310, 320. Afirst distal end of the air bridge structure 330 is attached to thefirst mounting region 312 of the first circuit element 310 and a seconddistal end of the air bridge structure 330 is attached to the secondmounting region 322 of the second circuit element 320 so as toelectrically interconnect the first and second circuit elements 310,320. In one aspect, the plane of the air bridge structure 330 maysubstantially parallel the plane of the substrate surface 304. It shouldbe appreciated that the plane of the air bridge structure 330 may varyin orientation, and the height at which the air bridge structure 330 issuspended may vary in magnitude without departing from the scope of thepresent invention.

As illustrated in FIG. 8, the electrical interconnect or air bridgestructure 330, in one embodiment, may comprise a laterally extendingmember 331, a first vertically extending leg 334, and a secondvertically extending leg 336. The laterally extending member 331 maycomprise a substantially planar structure that is substantially parallelto the substrate 302. The first vertically extending leg 334 may beinterposed between the first distal end of the laterally extendingmember 331 and the first mounting region 312 of the first circuitelement 310. The second vertically extending leg 336 may be interposedbetween the second distal end of the laterally extending member 331 andthe second mounting region 322 of the second circuit element 322. In oneaspect, the first and second vertically extending legs 334, 336 distallyextend from the substrate 302 in a substantially perpendicular mannerand form an electrical contact with the laterally extending member 331.It should be appreciated that the orientation and height at which thelaterally extending member is suspended above the substrate 302 may varyand depend on the length of the vertically extending legs 334, 336without departing from the scope of the present invention.

It should be appreciated that the electrical interconnect or air bridgestructure 330 may comprise the scope and functionality of the bridgesection 50 as described with reference to FIGS. 1-3 and may be formed ina manner as previously described without departing from the scope of thepresent invention. In addition, the electrical interconnect or airbridge structure 330 may comprise a conductive material, such as copperor silver, that may adapted to substantially improve theelectromigration as well as the electrical properties of the air bridgestructure 330.

As further illustrated in FIG. 8, the electrical interconnect or airbridge structure 330 may comprise a plurality of surfaces with exposedgrain boundaries 332. Grain boundaries may appear as irregularcrystalline lattice boundaries where grain interfaces coalesce. Theexposed grain boundaries 332 may be the result of using a CMP process toplanarize the upper surface of the air bridge structure 330. Otherwise,exposed grain boundaries 332 near the surfaces may be the result ofdeposition irregularities during metallization. CMP processes may causemechanical deformation of surfaces due to the applied mechanicalpolishing effects of rotating components used during the CMP process.Work-hardened materials tend to adversely affect the electricalproperties of crystalline structures.

As layers of material are polished away in a substantially uniformmanner, the surfaces become work-hardened and grain boundaries 332 areexposed near the surfaces. Beneficially, CMP process createssubstantially uniform surfaces, but CMP processes tend to exposesubstantially large quantities of grain boundaries 332 near thesurfaces. Unfortunately, an increase in the number of exposed grainboundaries 332 tends to increase surface diffusion and grain boundarydiffusion of impurities into the structure, which may detrimentallyeffect the integrity and purity of composition. When impurities diffuseinto the structure, a reduced integrity of composition results, whereinundesirable electrical properties may also result, which may lead to anincreased resistivity or reduced conductivity of the structure. Inaddition, the number exposed grain boundaries 332 may also lead toincreased surface electromigration and grain boundary electromigration,which may detrimentally effect the reliability of the device.

In one aspect, electromigration may be induced by an electric current inthe bulk material and may refer to the directed motion of atoms at solidsurfaces, grain boundaries, and grain interfaces. The Applicantconsiders electromigration as a key factor in determining thereliability of integrated circuits. As integrated circuitminiaturization continues and component densities increase, failures mayoccur when the interconnect line dimensions are relatively similar insize to or smaller than the grain proportions of the material. In oneembodiment, grain boundaries no longer provide connected diffusion pathsalong the conductive path. Instead, failure occurs due to intragranularvoids, which may nucleate at the edges of the conductive path orinterconnect path, migrate in the current direction, and collapse. Theproposed failures may also comprise diffusive displacements at theterminals of the interconnect line that may inhibit electrical contact.

Both of these failure modes may be affected by the microstructure of theinterconnect line and may be delayed or overcome by metallurgicalchanges that alter the crystalline microstructure. When electrons areconducted through a metal, they interact with imperfections in thelattice structure of the atoms and scatter. Scattering occurs wheneveran atom is out of place for any reason. Thermal energy producesscattering by causing atoms to vibrate. This may be considered thesource of resistance of conductive materials. The higher thetemperature, the more out of place the atom is, the greater thescattering and the greater the resistivity. Under these conditions,electromigration may lead to the electrical failure of interconnects inrelatively short times, which may reduce the lifetime of the integratedcircuit to an unacceptable level.

Advantageously, electromigration and diffusion may be deterred bycoalescing grain boundaries or re-crystallizing the crystallinemicrostructure of the electrical interconnect or air bridge structure330 in a manner that will be described in greater detail herein below.As is illustrated in FIG. 8, by allowing the structural composition ofthe air bridge structure 330 to re-crystallize 340, the quantity ofexposed grain boundaries 332 at the surfaces may be reduced. In oneaspect, the process of re-crystallization comprises a change in thegrain structure of a material during which the deformed grains, strainhardened by working, become new unstrained grains. Re-crystallizationpromotes grain development, wherein individual grains coalesce to formlarger and fewer grains. As the material re-crystallizes, minutecrystals may appear in the grains of the microstructure. These minutecrystals may comprise the same composition and lattice structure as theoriginal undeformed grains, which may comprise substantially uniformdimensions. The minute crystals may nucleate at the most drasticallydeformed portions of the grain, such as the grain boundaries. Thecluster of atoms from which the re-crystallized grains are formed maycomprise a nucleus. Re-crystallization takes place by a combination ofnucleation of the strain free grains and the development of thesenuclei.

In one aspect, the temperature at which material re-crystallizes and/orcoalesces is dependent on the characteristics of the material itself.The material used to form the air bridge structures may comprise, atleast in part, various metals, such as copper, silver, gold, platinum,palladium, iridium, and various reactive elements, such as titanium,zirconium, and hafnium or some combination thereof. It should beappreciated that, for example, if one of the reactive elements is used,zirconium may be preferred due to its low solubility in both copper andsilver. It should be appreciated by skilled in the art that the rate andtemperature of recrystallization depends not only on the material butalso the extent of prior cold work. Below is one embodiment of a tableof approximate re-crystallization temperatures for the above-mentionedmaterials.

Material Approximate Re-crystallization Temperature copper 200-400° C.silver 200-400° C. gold 200° C. platinum 400° C.

As a result of reducing the number of exposed grain boundaries near thesurface of the electrical interconnect or air bridge structure 330,improved electrical properties may be achieved from an increase in theelectromigration resistance and an increase in the diffusion resistanceof the recrystallized microstructure. To further enhance theelectromigration resistance and the diffusion resistance, the air bridgestructure 330 may be coated with an insulating material in a manner aspreviously described. Advantageously, improved electrical properties ofthe air bridge structure 330 may increase the reliability and structuralintegrity of the air bridge structure 330. Beneficially, by allowingre-crystallization to occur, a reduction in the quantity of exposedgrain boundaries is achieved.

FIGS. 9A-9H illustrate one embodiment of forming the electricalinterconnect 300 of FIG. 8 with enhanced electrical properties. Asillustrated in FIG. 9A, the substrate 302 may comprise an upper surfacewhere the first and second mounting regions 312, 322 are positioned in amanner as described in FIG. 8. FIG. 9B illustrates the deposition of asupport layer 350 that may be used to form a temporary supportstructure, such as the previously described mandril. The support layer350 may comprise an insulating material, such as silicon-dioxide, thatmay be globally deposited so as to overlie the substrate 302 and themounting regions 312, 322 in a generally known manner using depositiontechniques, such as chemical vapor deposition (CVD). In one embodiment,the support layer 350 may be planarly etched to a first height 370 usinggenerally known chemical-mechanical polishing (CMP) techniques.

FIG. 9C illustrates the formation of a first and second via 352, 354 anda mandril or temporary support structure 356 in the support layer 350.The vias 352, 354 may be formed using a generally known pattern and etchtechnique, wherein the first via 352 is etched in a manner so as toexpose the upper surface of the first mounting region 312, and thesecond via 354 is etched in a manner so as to expose the upper surfaceof the second mounting region 322. During etching of the vias 352, 354,the temporary support structure 356 may be formed using pattern and etchtechniques in a similar manner that is generally known. In oneembodiment, the temporary support structure 356 may be etched so as toretain the first height 370 as described with reference to the supportlayer 370 in FIG. 9B. Moreover, the temporary support structure 356 willprovide support for the laterally extending member 331 during depositionin a manner that will be described in greater detail herein below.

FIG. 9D illustrates the temporary support structure 356 in a modifiedform so as to define a second height 372 that is at least smaller thanthe first height 370. In one embodiment, the first height 370 of thetemporary support structure 356 may be reduced using generally knownpattern and etch techniques to the second height 372 so that the supportlayer remnants 350 retain the first height 370 and the temporary supportstructure 356 comprises the second height 372 as illustrated in FIG. 9D.

FIG. 9E illustrates the deposition of a conductive layer 358 that may beused to form the laterally extending member 331 and the verticallyextending legs 334, 336 of the air bridge structure 330. The conductivelayer 358 may comprise a conductive material, such as aluminum, copper,silver, or gold, that may be globally deposited so as to overlie theinsulation layer 350 including the temporary support structure 356. Theconductive material may also be deposited into the vias 352, 354 in asimilar manner. The conductive material may be deposited in a generallyknown manner using known deposition techniques, such as a chemical vapordeposition (CVD), plasma enhanced CVD (PECVD), vacuum evaporationelectroplating, or sputtering. As illustrated in FIG. 9D, it should beappreciated that, due to global deposition techniques, the conductivelayer 358 may comprise a non-planar upper surface.

FIG. 9F illustrates planar processing of the conductive layer 358 so asto form the laterally extending member 331 of the air bridge structure330. In one embodiment, a chemical-mechanical polishing (CMP) processmay be utilized to evenly planarize the non-planar surface of theconductive layer 358. The CMP planarization process applies asubstantially uniform material removal rate across the plane of thesubstrate surface 304, which substantially ensures that the conductivelayer 358 is uniformly reduced in height across the plane of thesubstrate surface 304 until the support layer remnants 350 are reached.Unfortunately, as previously described in FIG. 8, the planar processingof the conductive layer 358 may create work-hardened surfaces and exposegrain boundaries adjacent the upper surface of the conductive layer 358.Consequently, the planar processing may adversely affect the electricalproperties of the air bridge structure 330. Advantageously, the grainboundaries will be allowed to coalesce in FIG. 9G so as to improve thecrystalline structure, which improves the electrical properties of theair bridge structure 330.

FIG. 9F further illustrates one embodiment of forming the laterallyextending member 331 of the air bridge structure 330. As illustrated inFIG. 9F, a first distal end 333a of the laterally extending member 331forms an electrical contact with the upper portion of the firstvertically extending leg 334, and a second distal end 333b of thelaterally extending member 331 forms an electrical contact with theupper portion of the second vertically extending leg 336. It should beappreciated that the air bridge structure 330 electrically interconnectsthe first mounting region 312 with the second mounting region 322,wherein the formation of the laterally extending member 331 and thevertically extending legs 334, 336 forms the electrical interconnectionbetween the first and second mounting regions 312, 322 as illustrated inFIG. 9F.

FIG. 9G illustrates the removal of the support layer remnants 350 andthe temporary support structure 356 in a manner so as to leave the airbridge structure 330 intact. As illustrated in FIG. 9G, the laterallyextending member 331 is suspended above the substrate 302 via thevertically extending legs 334, 336 and positioned so as to laterallyextend between the upper portions of the vertically extending legs 334,336. In one embodiment, the support material may be removed in agenerally known manner using pattern and etch techniques including acidwashes.

Unfortunately, the removal of the support material may deform thesurfaces of the laterally extending member 331 and the verticallyextending legs 334, 336, which may adversely affect the electricalproperties of the air bridge structure 330. Therefore, at this point inthe formation process, the air bridge components 331, 334, 336 areprocessed in a manner so as to coalesce the exposed grain boundaries soas to improve the crystalline structure of the components 331, 334, 336and reduce the quantity of grain boundaries in a manner as previouslydescribed with reference to FIG. 8. Advantageously, as a result ofcoalescing the grain boundaries of the air bridge components 331, 334,336, the air bridge structure 330 has improved electrical properties,which results in a more reliable device as previously described in FIG.8.

FIG. 9H illustrates the formation of a adherent coating 360 on thesurfaces of the air bridge structure 330. The adherent coating 360 maybe deposited in a manner as previous described. It should be appreciatedthat the adherent coating may further enhance the electrical propertiesof the air bridge structure 330 in a manner as previously described. Itshould also be appreciated that the adherent coating 360 may comprisethe scope and functionality, including the dimensions and shape, of theadherent coating 62 as described with reference to FIGS. 4-7. Moreover,it should be appreciated that the adherent coating 62, in oneembodiment, may comprise a protective coating that serves as aprotective barrier which prevents surface diffusion as well asinhibiting contaminants, such as oxygen, from reaching the core 60.

Advantageously, electromigration and electron scattering may be deterredby coalescing grain boundaries and/or re-crystallizing the crystallinemicrostructure of the electrical interconnect or air; bridge structure330. By re-crystallizing the structural composition of the air bridgestructure 330, the quantity of exposed grain boundaries at the surfacesmay be reduced. In one aspect, the process of recrystallization maycomprise improving the grain structure of the conductive material duringwhich the deformed grains, strain hardened by planar processing, becomenew unstrained grains.

As preciously described, re-crystallization promotes grain development,wherein individual grains coalesce to form larger and fewer grains. Asthe material re-crystallizes, minute crystals may appear in the grainsof the microstructure. These minute crystals may comprise the samecomposition and lattice structure as the original undeformed grains,which may comprise substantially uniform dimensions. The minute crystalsmay nucleate at the most drastically deformed portions of the grain,such as the grain boundaries. The cluster of atoms from which there-crystallized grains are formed may comprise a nucleus.Re-crystallization takes place by a combination of nucleation of thestrain free grains and the development of these nuclei.

FIG. 10 illustrates one embodiment of a method 400 that maybe used toform the electrical interconnect or air bridge structure 330 asdescribed in FIGS. 8, 9. The method 400 initiates in a start state 402and proceeds to a state 404, wherein the mandril 104 or temporarysupport structure 356 may be formed on the upper surface 304 of thesubstrate 302 in a generally known manner as previously described inFIGS. 4, 9A-9H. The temporary support structure 356 may be placedbetween the first and second mounting regions 312, 322 so as to distallyextend above the surface 304. In one aspect, the purpose of thetemporary support structure 356 is to provide a supporting surface thatsubstantially supports the laterally extending member 331 duringformation of the air bridge structure 330. The temporary supportstructure 356 may be patterned, etched, and formed using variousmaterials and various deposition techniques that are generally known inthe art in a manner such that the temporary support structure 356 may beremoved to expose a lower surface of the air bridge structure 330.

After forming temporary support structure 356 in the state 404, themethod 400 advances to a state 406, wherein the temporary supportstructure 356 is modified, using conventional etching techniques, toform vias 352, 354 in the temporary support structure 356 thatvertically extend from an upper surface of the temporary supportstructure 356 to the respective first and second mounting regions 312,322. The correspond to the first and second vertically extending legs334, 336. Also, in the state 406, the height of the temporary supportstructure 356 may be modified so as to comprise a reduced height 372 ina manner as described with reference to FIG. 9D. Advantageously, thereduced height 372 of the temporary support structure 356 allows thelaterally extending member 331 of the air bridge structure 330 to beformed when the conductive layer 258 is deposited and planarized in amanner as described herein below.

Next, in a state 408, the method 400 proceeds to a state 410, whereinthe conductive layer 258 is formed in an overlying manner on thetemporary support structure 356 such that the conductive layer 258horizontally extends across the upper surface of the temporary supportstructure 356 between the vias 352, 354. The deposition of conductivelayer 258 is used to form the air bridge structure 330 including thelaterally extending member 331 and the first and second verticallyextending legs 334, 336. In one aspect, the conductive layer 358vertically extends through the vias 352, 354 to form the first andsecond legs 334, 336 and contact the first and second mounting regions312, 322 of the first and second circuit elements 310, 320. Theconductive layer 258 preferably comprises a highly conductive materialhaving a relatively small ratio of (ρ/E), such as copper or silver,thereby providing the air bridge structure 330 with a relatively smallresistance and a reduced tendency to sag as previously mentioned above.

The method then advances to a state 410, wherein the conductive layer258 is modified so as to define the shape of the air bridge structure330. For example, the laterally extending member 331 of the air bridgestructure 330 may be planarized using a conventional chemical mechanicalplanarization (CMP) process. As previously described, the CMP process isused to remove material across a surface in a substantially uniformmanner. As is known in the art, the formation of substantially uniformsurfaces are desirable for the subsequent deposition of additionallayers with uniform thickness.

Next, the method 400 proceeds to a state 412, wherein the temporarysupport structure 356 is removed in a generally known manner so as toexpose the lower surfaces of the air bridge structure 330. Once thetemporary support structure 356 is removed in the state 412, theconductive layer 258 material of the air bridge structure 330 is allowedto re-crystallize in a state 414. According to the Applicant, somematerials that may be used to form the air bridge structure 330 mayrecrystallize at room temperature. For example, copper following a CMPprocess may re-crystallize at 200° C. In contrast, for example, acopper-tin(0.24%) alloy may require a heat treatment above 375° C. forre-crystallization.

In one aspect, a pre-determined time allotment may granted for thepurpose of microstructure re-crystallization of the material used toform the air bridge structure 330. In another aspect, it may bedesirable to utilize environmental control techniques to controlmicrostructure re-crystallization. For example, the integrated circuit300 including the air bridge structure 330 may be placed in anenvironment conducive to allowing desirable re-crystallization. Theenvironment may comprise a vacuum, wherein contaminants and impuritiesare removed from the atmosphere. Other factors that may influencedesirable re-crystallization may include temperature and pressurecontrol, wherein various heat treatments and pressure treatments may beutilized to control microstructure re-crystallization. It should beappreciated that the air bridge structure 330 may comprise similarfeatures including dimensions, shape, and functionality as describedwith reference to the bridge section 50 as reference by FIGS. 4-7.

After allowing microstructure recrystallization in the state 414, themethod 400 advances to a state 416, wherein disposing of a adherentcoating on the exposed surfaces of the electrical interconnect may beperformed. In one aspect, disposing the adherent coating may comprisedepositing a layer of conductive material selected from the groupcomprising the noble metals gold, platinum, palladium, iridium, and thereactive elements titanium, zirconium and hafnium. Furthermore, theconductive material of the coating may be deposited using a knownelectroless plating process or a known CVD process. It should beappreciated that, for example, if one of the reactive elements is used,zirconium may be preferred due to its low solubility in both copper andsilver.

In one embodiment, disposing the adherent coating may comprisedepositing an insulating material. For example, the insulating materialmay comprise an organic material, such as parylene, which can bedeposited using a known vapor deposition polymerization process.Alternatively, the insulating material can comprise an inorganicmaterial, such as Si₃N₄, which can be deposited using PECVD. In oneaspect, diffusion of the adherent coating may be inhibited by notexposing the air bridge structure 330 to elevated temperatures.Preferably, the adherent coating is deposited in a manner such that thematerial only deposits on the air bridge structure 330. It should beappreciated that the adherent coating may comprise similar featuresincluding dimensions, shape, and functionality as described withreference to the coating 62 as reference by FIGS. 4-7. It should also beappreciated that, after applying the adherent coating on the air bridgestructure 330, a heat treatment may be used to improve adhesion alongthe grain boundaries 332 on the surfaces of the air bridge structure330, which may further enhance the electromigration resistance of theair bridge structure 330. After depositing the adherent coating on theair bridge structure 330 in the state 416, the method 400 proceeds toterminate in an end state 418.

In one embodiment, mechanical stability of the air bridge structure 330may be increased by back-filling the spaces or vacancies at least inpart with an insulating material between the air bridge structure 330and the substrate 302. In addition, a foamed polymer may be used in amanner as disclosed in the Applicant's issued patent entitled “Method ofForming Foamed Polymeric Material for an Integrated Circuit” (U.S. Pat.No. 6,077,792), which is hereby incorporated by reference in itsentirety. The issued patent discloses a method of forming an insulatingmaterial, such as a polymetric material, for use in an integratedcircuit, wherein at least a portion of the polymeric material isconverted to a foamed polymeric material. The converting of thepolymeric material includes exposing at least a portion of the polymericmaterial to a supercritical fluid. The integrated circuit may include asubstrate of the integrated circuit and a foamed polymeric material onat least a portion of the substrate. The integrated circuit may furtherinclude a conductive layer adjacent the foamed polymeric material.

By allowing the microstructure of the electrical interconnect or airbridge structure 330 to re-crystallize in the state 414, the quantity ofexposed grain boundaries may advantageously be reduced. A reduction inthe quantity of exposed grain boundaries may lead to an improvement inthe electrical properties, including enhanced electromigrationresistance and enhanced diffusion resistance, of the electricalinterconnect or air bridge structure 330. Enhanced electrical propertiesmay improve the reliability of the electrical interconnect or air bridgestructure 330 by improving the crystalline orientation and compositionalintegrity of the microstructure. Although the preferred embodiment ofthe present invention has shown, described and pointed out thefundamental novel features of the invention as applied to thisembodiment, it will be understood that various omissions, substitutionsand changes in the form of the detail of the device illustrated may bemade by those skilled in the art without departing from the spirit ofthe present invention. Consequently, the scope of the invention shouldnot be limited to the foregoing description, but should be defined bythe appending claims.

1. A method of forming an air bridge structure between a first andsecond circuit component on a substrate, the method comprising: forminga support structure on the substrate; forming vias in support structureabove the first and second circuit components; depositing a conductivelayer so as to form vertically extending legs in the vias and alaterally extending member between the upper portions of the verticallyextending legs in a manner so as to form an air bridge structure whichelectrically interconnect the first and second circuit components,wherein forming the laterally extending member results in an increasedresistivity through the air bridge structure; removing the supportstructure so as to suspend the laterally extending member above thesubstrate between the first and second circuit components via thevertically extending legs, wherein removing the support structureresults in an increased resistivity through the air bridge structure;and processing the air bridge structure by recrystallizing the laterallyextending member and the vertically extending legs, which results in adecreased resistivity through the air bridge structure.
 2. The method ofclaim 1, wherein depositing the conductive layer includes depositing amaterial with a line resistivity of at least less than that of aluminum.3. The method of claim 1, wherein depositing the conductive layerincludes depositing a material with a ratio of mass density over modulusof elasticity (E/ρ) that is at least greater than gold.
 4. The method ofclaim 1, wherein crystallizing the laterally extending member comprisescoalescing a plurality of grain boundaries in a manner so as to formfewer grain boundaries.
 5. The method of claim 4, wherein coalescing thegrain boundaries occurs at room temperature.
 6. The method of claim 4,wherein coalescing the grain boundaries includes performing a heattreatment.
 7. The method of claim 4, wherein coalescing the grainboundaries improves the electrical properties of the conductive layer.8. The method claim 7, wherein improving the electrical properties ofthe conductive layer includes enhancing the electromigration resistanceof the conductive layer.
 9. The method claim 7, wherein improving theelectrical properties of the conductive layer includes enhancing thediffision resistance of the conductive layer.
 10. The method of claim 1,wherein the method further comprises annealing the air bridge structure.11. The method of claim 1, wherein forming the laterally extendingmember includes planarizing the conductive layer using a CMP process.12. The method of claim 1, wherein forming a conductive layer comprisesdepositing a least one of copper, silver, and gold.
 13. The method ofclaim 1, wherein the method further comprises forming an adherentcoating on the air bridge structure.
 14. The method of claim 13, whereinforming the adherent coating on the air bridge structure includesdepositing at least one of titanium, zirconium, and hafnium on the airbridge structure.